1. Field of the Invention
This invention relates to a diversity reception control circuit for performing diversity reception control in a radio communication apparatus which performs diversity reception.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a configuration of a diversity receiver which performs diversity reception. In FIG. 1, reference numerals 11a and 11b are antennas which are installed at a distance from each other, 12a and 12b are high frequency amplyifying circuits which extract and amplify a desired band and have a filter function, 13a and 13b are mixers which mix output signals of the high frequency amplifying circuits 12a and 12b and a signal output from a local oscillator 14, respectively, 15a and 15b are intermediate frequency circuits which select a necessary frequency band from an intermediate frequency signal which the mixers 13a and 13b output, and produce demodulation outputs 23 and 24, respectively, 16a and 16b are detection circuits which detect outputs of the intermediate frequency circuits and produce output DC signals 21 and 22, and 30 is a diversity control circuit which selects one of the demodulation outputs 23 and 24 and outputs the selected demodulation output 25. The distance between the antennas 11a and 11b is selected at approximately one-half wavelength of the carrier frequency.
Next, the operation will be described. Radio signals which have been inputted to the antennas 11a and 11b are amplified by the high frequency amplifying circuits 12a and 12b in their desired bands and are inputted to mixers 13a and 13b, respectively. The input signals are converted into intermediate frequency signals by signals from the local oscillator 14. The intermediate frequency signals are inputted into the intermediate frequency circuits 15a and 15b where they are converted into demodulation outputs 23 and 24 of desired channels by selective filters (intermediate frequency filters). One part of the output signals of the intermediate frequency circuits 15a and 15b is subjected to detection such as envelope detection by the detection circuits 16a and 16b, respectively, and converted into the DC signals 21 and 22 proportional respectively to the input signal strengths in each of the receiving systems. Hereinafter, the DC signals 21 and 22 are respectively referred to as RSSI 21 and RSSI 22. (RSSI: Received Signal Strength Indicator). Also, RSSI 21 and RSSI 22 are collectively represented by RSSI 21 and 22. These signals RSSI 21 and 22 vary by undergoing the influences of fading. The levels of the are compared by a comparator which is built into the diversity control circuit 30. The one of the demodulation outputs 23 and 24 which has a higher level is regarded as produced by the antenna located at a spot where the receiving electric field strength is larger, and the higher level one of the demodulation outputs 23 and 24 is selected and outputted from the diversity control circuit 30. It is known that, for an equivalent receiver, sensitivity based on such a diversity reception as described above is improved by 5 to 6 dB in mobile radio communications of a digital system compared with that in the case where diversity is not used. This corresponds to improvement of one to two orders of magnitude in terms of bit-error-rate (BER), which is the error rate in the demodulation of the digitally encoded data. This is illustrated in FIG. 4 and will be explained later in more detail.
Since a conventional diversity control circuit is constituted by a comparator for comparing RSSI levels as described above, the characteristics of each receiving system (antenna, high frequency amplifier, mixer, and intermediate frequency circuit) and those of each of the detection circuits 16a and 16b are desirably indentical in order to provide the same input/output characteristics with respect to each of the RSSI signals 21 and 22 in relation to receiving electric field strength. But, actually, the RSSI signal vs. receiving electric field strength characteristics do not coincide in many cases as shown in FIG. 2. For instance, in region A in FIG. 2 and as shown in enlarged format in FIG. 3, it is possible that the RSSI signals 21 and 22 are generated in inverse proportion to respective actual receiving electric field strengths. Where the values of the RSSI 21 and RSSI 22 are x.sub.1 and x.sub.2 as shown in FIG. 3, the diversity control circuit 30 selects the demodulation output 23 corresponding to the RSSI 21 because that is the higher level signal, even though the receiving electric field y.sub.1 for RSSI 21 is less than that of y.sub.2 for RSSI 22. This means that the diversity control circuit 30 has selected the demodulation output 23 corresponding to the weaker receiving electric field strength because y.sub.1 is less than y.sub.2. Thus, when the RSSIs 21 and 22 are compared by a simple comparator, it is possible to cause selection of the demodulation output of the radio signal for the one of the antennas 11a and 11b which is located at a spot where the receiving electric field strength is actually lower.
Also, since characteristics of the detection circuits 16a and 16b begin to saturate relative to their inputs in regions where the receiving electric field strength is large, the selection between the demodulation outputs 23 and 24 can be further biased toward either signal, such that the advantage of the diversity effect is further lowered, such deterioration being shown by a dotted line in FIG. 4.